Secret signaling



R. V. L HARTLEY SECRET SIGNALING Aug. 7, 1923. 1,464,096

Filed Aug. 28, 1920 Rev/ 0h V. L Har7/e which the following ATENT. orrics mrn "v'; n nanr'migor vEasafloxalic/1:, NEW JERSEY, ASSIGNOB-TO wns'rnnn ynnnornrc comranxmconronarnn, or new YORK, n. Y., a-oonronarron or new YORK.

- SECRET SIGNALING.

Application mem s-1a as, 1920. swam; 406,700.

To allwho-mliimayconcern: I

Be itk'nown that I, RALPH V. L. HARTLEY,

a citizen of the United "States, residing at East Orange,- in thecounty of Essex, State of New Jersey, have invented certain newand useful Improvements in Secret Signaling, of is a 'full, clear,- concise,

and exact description.

' The present invention relates to the control of waves employed in signaling or in the j transmission of ener 'for other purposes.

.M ore specifically, in el invention relates to the control of carrlerwaves and to the modul latijon of such waves inaccordance with sig- -18 -such waves.

' cerned in the modulation and control of car-- rier waves used insecret signaling, particu-. larly in secret telephony. 20,

'nals, and to transmission and reception of The invent1on 1s especially conln'an'application of Burton W. Kendall, Serial No.409,073, filed September 9,1920,

3. system of secret telephony isdisclosed' inwhich a high frequency wave of varying fre que'ncy is used for the carrier. In that system, which, like the one presently to be described, iszapplicable to use'on Wire line transmission systems, the unmodulated carr-ier-wavec'omponent is sup ressed, the curaresuppressed. and one side band only of the 1 mod-ulated wave is allowed to reach the line. 'Thecomplete suppression of the undesired ulator.

frequency components is accomplished by plural modulation and b filters, and does not depend upon any particulartype of mod To receive the wave of such a system, it is necessary to supply tothe. receiving.

circuit awave havingthe characteristics of the carrier wave and unless the frequency,

and the scheme of varying the frequency'of the carrier wave are known, it is impossible to receive the message transmitted.

The preferred manner 'of. controlling the Currents of essential voice frequencies 8 (or other signaling frequencies) modulate a wave offixed frequency p, giving, as one component, current of-frequencies p+s. A wave of continuously varying-frequency, indicated by p+r+d where o' is 'a fixed frequency and d varies in any predetermined manner. between zero and a chosen fixed 11m1t',-1s'modulated :by the component current of frequency p-hs giving as a result a 'm'odulated. wavehaving a lower side fre-' uency component of frequency v+d-.s, and

t eother componentsof which are of higher.

frequency than p. If, as in the case of voice currents, 8 comprises a'band'offrequencies,

ther'elwill be a bandof frequencies 7) {d-8.

Since p can-be made arbitrarily large, it is a signed limit, to the-exclusion of the components of all the other frequencies present,

which are suppressed by the filter.

The frequency 'cH-cZ-may be within the limits of audibility although this is not cs sential, In such-a case a side band of frequency o+(l-s will be audible. A particular component of this side band if produced by a low frequency speech current will be of comparatively high frequency; if by ahi h frequency speech current, of comparativey low frequency. The currents of the resulting side band, although of audio frequency, will have an entirely different frequency arrangement from the original speech currents and will be unintelligible. It is therefore possible to transmit unintelligible message currents'and the system may in this manner be given an additional degree of secrecy.

Thecarrier frequency used in such a system I is 0+0? and the frequency p does not appear in the final modulation product that is transmitted. The wave of frequency ;0 serves only in the intermediate step of the modulation in order to step up, the effective frequenc of thespeech to the value p+s to give nal modulation -components differing widely in frequency and easily capable of separation by filters. The carrier frequency is therefore derived as the difference between two waves of frequencies p and p+o+d respectively;-

If the waves p and p-l-o-i-(l are derived from independent sources, the difference wave will not in'practice be exactly 41+d where d indicates thepredetermined variable component of the carrier frequency, but

mitting and rece1vin ently so as either to maintain their frequencies absolutely the same or to have their frequency variations exactly alike If the frequency p of the wave. which 18 introduced merely to take part in the production of the wave to be transmitted,but is not itself transmitted, is always maintained equal to the component 9 in'the wave p+2+d which is to be neutralized in the second stage of the modulation then the carrier wave will be exactly o-i-d. According to the present invention, the Waves of frequencies p and n+0! are separately generated and in the modulating process at the transmitter they are added together and thereafter the same wave of frequency ;0 is subtracted again so that the resulting frequency o-i-ai is maintained independent. cf variations in the frequency 32.

The nature and objects of the invention will appear more fully from the following description of a specific embodiment of the invention as illustrated in the schematic circuit diagram of the accompanying drawing.

This circuit diagram represents the apparatus at one station. on the line 1 and in cludes the necessary elements for both transsecret telephone messages. The line 1 1s balancedby the net 2 and the coils 3 and 4 in the customary manner to permit two-way communication over the line without interference between the transmitting and receiving circuits. At

.the station illustrated, the transmitting apparatus is shown in the upper portion of the figure and the receiving apparatus in the lower portion.

The wave which serves as a carrier for the speech currents is generated by the oscillator'5 which may be of any suitable type and is shown as an oscillating thermionic tube. The fre uency of theoscillator is controlled by t e resonant circuit 6. the tuning of which is preferably cyclically varied by means of the variable condenser 7 which has a movable plate or plates continuously driven by the constant speed motor 8. Any other means of producing a wave of varying frequency -'v+d may be used. Although this Wave has been previously'described as preferably of audible frequency, it maybe of very high or inaudible frequency' It is assumed that a similar oscillator similarly controlled to give a frequency v-l-d is located at the distant receiving station, since in the present system such a wave must be used at the receiver to render the transmitted message intelligible. The motor-drivencondenser at that station (not shown) is preferably kept in synchronism out periodically from a contact-making means 9 operate to introduce base corrections in the motor-driven con enser at the distant station. It is also evident that these impulses give no clue to the carrier frequency used or to the scheme of varying it,

since any desired gear ratio maybe used between the motor and the condenser, or the condenser plates can have any desired number and shape of scallops, and the number of impulses sent out per revolution by the motor can be varied to suit conditions. The inductances 10 and condenser 11 serve as a low frequency filter to permit the synchronizing impulses to pass to the line but to keep the signaling frequency currents out of the commutator circuit.

The wave of high frequency p which is used in the preparation of the wave for transmission is generated by the thermionic oscillator 12 which may be of any suitable type for producing a continuous high frequency wave. The out-put waves of frequencies p and 0+0?) from the. oscillators 12 and 5 respectively, are impressed on the input circuit of the modulator 13. Potentiometer resistance 14 and 15 enable the amplitude of the respective waves applied to the modulator to be adjusted. By the wellknown action of the modulator 13, currents of combination frequencies involving the impressed frequencies are produced including an upper side band of frequencies p+r+d and other components of frequencies lower than this. The high pass filter 16 is designed to permit the passage of the frequencies comprised in the band p-l-o-l-d but to suppress all the other frequency components from the out-put of the modulator. The wave of frequency p+c+d isthen applied through the coupling 17 to the input circuit of the modulator 18.

The oscillator 12 in addition to supplying the waveof frequency -,v to the modulator 13 also applies a wave of this frequency to the modulator 19. The amplitude of the wave so applied may be controlled by the potentiometers 15 and 20. This wave is modulated in the tube 19 by the signal waves such as speech waves originating in the microphone circuit 21. A low-pass filterBQ suppresses currents of the unessential high frequencies of the voice and permits the passage of all frequency currents up to a predetermined upper limit. As a result of this modulation, an upper side band of frequencies p+s will be obtained in addition to fre uencies 9, 77-8, and components of the dou 1e frequencies of both 1) and 8' and possibly still other frequencies. The filter 23 which is a high pass filter is designed to transmit the upper side band p-i-s but to suppress all lower frequency components. Currents comprising the band p+s are therefore supplied to the input circuit of the modulator 18 along with the wave p-I-v-l-d applied through the coupling 17 as described. Among the most important frequency components resulting from the action of modulator 18 are the upper band of frequency p+s+p+v+d, the lower band of frequency p+v+a3- (p-l-s) :cH-d-s, and the unmodified component of frequency p-l-s. Since 7 can be made arbitrarily high, it is evident that the lowest frequency in the band p-f-s can be made as far as desired above the highest frequency in the band, cr+ds and this band v+ds may therefore be completely separated by the low pass filter 2st and transmitted to the line.

Since the wave of frequency, p supplied to the modulator 19 always has the same frequency as the wave su plied to the modulator 13 to produce the Frequency p+v+d, it is evident that the difference frequency e+d will be at all times unaffected by fluctuations in the frequency of the oscillator 12.

It is assumed that the wave received at the station shown over the line 1 has a frequency o+is, although if desired, a different frequency could be used for transmitting in this direction. The wave to be received is selected by the filter 25, is acted upon by the amplifier 26, and is impressed on the detector 27 through the coupling 28. To receive the signal component 8 of this wave, it is necessary to supply to the detector 27 a wave of frequency n+4. This 1 wave of frequency a-l-d is derived from the oscillator 5 in the same manner as is the wave used in transmitting, and its strength may be adjusted by the potentiometers 1i and 29. The tube 26, in addition to amplifying the received wave, prevents the passage of the wave r+d from the oscillator 5 to the line by Way of the input circuit of the detector 27 and coupling 28. The two Waves impressed on the detector combine to reproduce the signaling wave 8 which is selectively transmitted by the low pass filter 30 and gives an audible signal in the telephone receiver 31.

It will be seen from the foregoing description that by means of the present invention the wave of high frequency 79 can be introduced and later can be completely neutralized so as not to produce disturbing freuencies in the final output. This introductlon of the high frequency Wave and its elimination after it is used, is especially applicable to the system of secret telephony of the type disclosed but may also have application in other kinds of systems as well.

The invention therefore is not to be construed as limited to the particular system which has been illustrated and described but its scope is defined in the appended claims.

\Vhat is claimed is:

1. The method of modulating a carrier wave by a signal which comprises supplying another wave, modulating said other wave by said signal, modulating said other wave by the carrier wave, and modulating one of the components resulting from the modulation of said other Wave by the carrier wave by a component resulting from the modulation of said other wave by said si nal, to produce a carrier wave'modulated signal.

2. The method of producing a carrier wave modulated by a signal which comprises supplying a wave of different fre quency from said carrier wave, modulating said wave of different frequency by said signal, separately modulating said wave of different frequency by said carrier wave, and combining in a modulator a component resulting from one such modulating step With a component of the other modulating step to produce a carrier ware modulated by said signal.

3. In a modulating system employing two stages of modulation, the method of supplying to one stage a high frequency wave and to the other stage another high frequency Wave having a frequency equal to the sum of the carrier frequency to be used and the frequency of said first-mentioned wave, which method comprises producing a wave of the carrier frequency and a wave of said first-mentioned high frequency. supplying said wave of first-mentioned high frequency to one stage, separately modulating said wave of first-mentioned high frequency by said carrier wave to produce their sum frequency, and selecting and transmitting to said second stage the wave of said sum frequency.

4. In modulated wave signal transmission, the method of producing a pure-modu lated Wave of a definite carrier frequency which comprises modulating a different high-frequency wave by the signal to be transmitted, modulating said high frequency wave by the carrier wave. selecting the sumfrequency components produced by each such modulation, modulating one of said sum-frequency components by the other to produce a signal-modulated carrier wave, and selecting for transmission a portion of the signal-modulated wave components comprising one side baud only of said signal modulated wave.

5. In a transmission system means for producing a carrier wave, means for producing another wave of a different frequency, means for modulating said Wa e 0f by said I different frequency by signals to be transmitted and for modulating said .wave of different frequency by said carrier wave to produce their combination fre uencies, and means for selecting and for mo ulating certain of said combination frequencies by the signal-modulated wave of said different frequency to produce a signal-modulated carrier wave. A

6. In a transmission system, a generator of a hi h frequency wave, a generator of a wave of a different frequency, a modulator for controlling one of said waves by the other to produce their combination frequencies, means for selecting certain of said combination frequencies, two modulators in tandem for signal transmission, and means for supplying to one of said modulators, a portion: of the output wave of one of said generators and for supplying to the other-of said modulators the selected combination frequencies.

7. A secret telephone transmission system comprising a modulator and circuits for supplying to said modulator currents of essential voice frequencies and a wave of constant frequency to be modulated thereby, a circuit for selecting and transmitting the upper side band resulting from such modulation and for suppressing all lower frequency components, a source of waves of varying high frequency, a second modulator and circuits for impressing on said modulator the currents offrequencies comprising said upper side band and a wave of varying high frequency produced by the combination of said wave of constant high frequency with the wave from said source of varying high frequency, and a circuit for selecting and transmittin currents of the frequencies comprising the ower side band from second modulator and for suppressing transmission of unmodulated currents of both said constant frequency and said varying high frequency.

8.-The method of modulating a carrier wave by a modulating wave, which comprises increasing the frequency of the modu-.

lating wave and increasing the frequency of the carrierwave by equal amounts before modulation, whereby the frequency separation of the resulting side bands is increased and the effective transmission carrier fre'-- quency is unchanged.

9. The method of modulating a carrier wave by a modulating wave, which comprises combining a third wave with said modulating wave and said carrier wave to increase the frequency of both said carrier and said modulating wave by the frequency of said third wave, and modulating the in creased frequency carrier wave by the increased frequency modulating wave.

10. A modulating system comprising a source of carrier waves, a source of modulating waves, means for increasing the frequency of said carrier waves by a definite amount, means for increasing the frequency of said modulating waves by the same amount, and means for modulating said increased frequency carrier wave by said increased frequency modulating wave.

11. In combination, means for producing a modulating wave, means for producing a carrier wave, means for producing a third wave, means for combining said third wave with said carrier wave to produce a carrier wave of increased frequency, means for combining said third wave with said modulating wave to produce a modulating wave of increased frequency, and means for modulating said increased frequency carrier wave by said increased frequency modulating wave.

12. The method of producing and transmitting. a signal-modulated carrier wave while completely suppressing transmission of both the signaling waves and the carrier waves themselves, comprising separately combining the signaling wave and the carrier wave with a third wave to obtainvmodulated waves of respectively higher frequency than the signaling wave or the carrier wave, transmitting these modulated waves while suppressing transmission of the signaling and carrier frequencies by filtering and modulating these two separately produced modulated waves by each other to produce a signal-modulated carrier wave for trans mission.

13. In combination two modulators, a common source of sustained waves applied to both of said modulators, a source of signaling waves applied to oneof said modulators, a source of carrier waves applied to the other, filters connected to the output of said modulators for selectively passing certain of the modulated components only and a third modulator suppliedthrough said filters.

14. In a transmission circuit three separate modulating circuits, a source of waves applied in common to two of said modulating circuits and an individual source of waves applied to each of said two modulating circuits, said third modulating circuit being supplied with two waves through said two modulating circuits. I

15, In a transmitting circuit a source of signaling waves, a source of carrier waves and a modulator for effecting modulation of said carrier waves by said signaling waves, a modulator interposed between said first modulator and said source of signaling waves, another modulator interposed between said first modulator and said source of carrier waves and a source ofwaves connected in common to .said two interposed modulators.

16. In combination two modulators supplied by sustained waves of the same frequency and each b a different modulating wave, and a thir modulator su plied by output waves from said two modu ators.

17. Means for modulating a carrier wave by signaling waves of nearly the same frequency as said carrier wave while completely suppressing transmission of unmodified components of both the carrier and the signaling waves, comprising a modulator, high ass filters connected between said moduator and the source of signaling and carrier waves, the characteristics of said filters being such that they suppress transmission of currents of the order of frequency of 'said signaling waves and of said carrier waves, frequency-changing circuits for elevating the frequency of both said signaling and said carrier waves by the same amount so that the waves of elevated fre uency pass through said filters and into sai first mentioned modulator and a filter connected to the output of said first mentioned modulator for transmitting only the lower side band of the modulated waves.

18. In combination a source of signaling waves and a source of carrier waves, means to increase the frequency of each of said waves by the same amount, filters for selectively transmitting the increased frequency waves, said filters suppressing transmission of currents of the frequencies of said sources, a modulator supplied by the increased frequency waves from said filters and a low pass filter connected to said modulator for transmitting only the difference frequency between the frequencies of the applied waves.

19. In secret signaling the method of modulating a carrier wave by speech waves while preventin the passage of either the carrier wave or %he speech waves themselves to the'line through the modulating appawhile suppressing transmission of all lower frequency waves including the original speech and carrier waves, modulating one of said upper side band waves by the other and selectively transmitting the lower side band resulting from said last mentioned modulation.

20. In a signalin system a source of signaling waves of r equency 8, other wave sources of frequencies 1) and 'v-i-d respectively, means to produce by modulation waves of frequencies p+s and p+v+d res ectively, filtering circuits for separating t ese modulated waves practically ure from waves of lower frequencies, an further modulatin and filtering means for producing from t ese modulated waves a wave of frequency 'u+ds to the substantial exclusion of waves of all other frequencies.

'In witness whereof, I hereunto subscribe my name this 13th day of August A. D.,

RALPH V. L. HARTLEY. 

